I may have spoke too soon [1].
Smirk called and said he was experiencing DSL (Digital Subscriber Line) issues—his connection dropped several times today for up to a minute or so. I told him it might be a port configuration problem (which is what caused the problems on Wednesday [2]) and that I would talk to Dan the Network Engineer about it (Dan being the expert on the type of router we use for our core).
But I took a look, and from what I can see, it may not be a port issue but an OSPF (Open Shortest Path First) issue:
>
```
May 01 00:07:12 XXXXXXXXXXXXXXXXXXXXXXX %OSPF-I-ADJCHGFROMFULL, Process 0, Nbr XXXXXXXXXXXXX on interface XXXXXXXXXXXXX changed from state FULL
May 01 00:07:12 XXXXXXXXXXXXXXXXXXXXXXX %OSPF-I-ADJCHGTOFULL, Process 0, Nbr XXXXXXXXXXXXX on interface XXXXXXXXXXXXX changed to state FULL, Loading Done
May 01 00:09:25 XXXXXXXXXXXXXXXXXXXXXXX %OSPF-I-ADJCHGFROMFULL, Process 0, Nbr XXXXXXXXXXXXX on interface XXXXXXXXXXXXX changed from state FULL
May 01 00:09:36 XXXXXXXXXXXXXXXXXXXXXXX %OSPF-I-ADJCHGTOFULL, Process 0, Nbr XXXXXXXXXXXXX on interface XXXXXXXXXXXXX changed to state FULL, Loading Done
```
The interface in question is the one between the core router and the DSL router. It doesn't seem to have anything to do with one of the OSPF routers participating is at the end of an authenticated PPPoE connection over DSL since the only complaints seem to be isolated with the connection between the core router and the DSL router.
Looks like I'll be doing some heavy reading in the next day or so.